This application is based upon and claims priority from Japanese Patent Application No. Hei. 10-72369 filed Mar. 20, 1998, the contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a gas concentration sensor for detecting a concentration of a specific constituent in a detected gas, for example, oxygen concentration in an exhaust gas from a vehicle engine, and specifically, to a method and apparatus for detecting element resistance of the gas concentration sensor.
2. Description of Related Art
A conventional oxygen concentration sensor (i.e., a limit current air-fuel ratio sensor) detects an oxygen concentration in exhaust gas from a vehicle engine. This sensor outputs a current signal relative to the oxygen concentration in the exhaust gas when a voltage is applied thereto. As to this gas concentration sensor, it needs to detect an internal resistance of the solid electrolytic (element resistance) accurately to detect its activation state or deterioration state.
FIGS. 10A and 10B are waveforms of voltage change and current change according to a conventional element resistance detection procedure of the gas concentration sensor, as disclosed in, for example, Japanese Laid-open patent application No. Hei. 9-292364. As shown in FIG. 10A, first, a voltage having a given time constant is applied to the gas concentration sensor. Next, a peak current xcex94Ixe2x80x2, which is an amount of current change, is detected after passing tTHxe2x80x2 time. The element resistance is detected based on the peak current xcex94Ixe2x80x2 and a voltage xcex94Vxe2x80x2 which is an amount of voltage change during tTHxe2x80x2 time. The element resistance corresponds to (xcex94Vxe2x80x2/xcex94Ixe2x80x2). In this detection procedure, the voltage applied to the sensor change with respect to the given time constant so that a generation of excessive peak current can be prevented. Therefore, this sensor can detect both a sensor current an element resistance of sensor.
However, detection of the element resistance may fluctuate due to variation of an element resistance detection parameter, such as a fluctuation of the time constant of the voltage applied to the sensor. A mechanism of the above is explained hereinafter with reference to FIGS. 11A, 11B. As represented by the two-dot chain line in FIGS. 11A, 11B, a changing speed of the voltage fluctuates unexpectedly when the time constant of the voltage fluctuates. In this situation, a time required to reach to the peak current xcex94Ixe2x80x2 also fluctuates. Then, the current detected after the tTHxe2x80x2 time may not always reach to the peak current xcex94Ixe2x80x2. As a result, an accuracy of element resistance detection decreases.
In order to solve the foregoing problem, the present invention provides an element resistance detection procedure of a gas concentration sensor that detects an element resistance accurately even if some parameters, such as a time constant or a detection timing of current change due to voltage change, fluctuates. The present invention also aims to provide a gas concentration detection apparatus which employs the above element resistance detection procedure.
More particularly, the element resistance detection procedure of the present invention is characterized in that timings for detecting the voltage change and current change are set before the voltage, which having a given time constant, converges to a constant value. Also, the detection procedure of the present invention can detect the element resistance accurately even if some parameters, such as the time constant or the detection timing of the current change due to the voltage change, fluctuates.
The gas concentration detection apparatus of the present invention includes a first circuit to detect whether an amount of a voltage having a given time constant reaches a given value, a second circuit to determine a detection of voltage change and current change used for an element resistance detection, at the time of detection of the first circuit, and a third circuit to detect the current change in relation to the voltage change.
The gas concentration detection apparatus can detect the element resistance accurately even if some parameters, such as the time constant or the detection timing of the current change due to the voltage change, fluctuates. Since this apparatus is formed by electric circuits (i.e., the first, second and third circuit), a timing which the amount of the voltage change reaches the given value can be detected accurately.
In other words, in an engine control microcomputer (i.e., engine control ECU), operation programs are executed at a given cycle, and various other programs are executed. Therefore, the detection timing of the voltage change and the current change may fluctuate by at least 10 xcexcs, and the detected element resistance may include some errors.
On the other hand, according to the present invention, the voltage change and the current change can be detected instantly at a desired timing, and fluctuation of the detection timing of the voltage change and the current change can be decreased substantially. Consequently, the element resistance can be detected accurately. Furthermore, since the voltage change and the current change are instantly detected, the detection impossible period of the gas concentration (i.e., A/F) can be decreased.